Musings On Air Quality

Lately I’ve been pondering the question of winter indoor air quality here at the Cold House, spurred a bit by posts by Crunchy Chicken, who is working on a book about environmental toxins. Also spurred by the observation that when we cook something smelly for dinner, it stays smelly-smelling downstairs for a long time.

To be honest I’m not overly concerned about natural hazards (mold, mites, very small snakes), because (a) we don’t seem to be very sensitive to such things, and (b) they are naturally minimized by cold temperatures. I’m more worried about volatile chemical substances, including phthalates and other bizarre hydrocarbon substances such as might come from vinyl shower curtains, scented candles, burned food, cleaning products, etc. If any of these are present, they’re definitely sticking around longer in winter than in summer when all the windows are open.

The wood stove you might think to be a big offender in this mix, and certainly some I’ve used in the past were prone to filling the house with carcinogenic-smelling smoke. Often you knew you were in a wood-burning house as soon as you walked in the door. Our new Jøtul, however, is so well-designed that it virtually never gives even a hint of smoke odor. In fact, the stove is probably the #1 positive force for good air quality in the house: it pulls old air up the chimney, and fresh air sneaks in through cracks to replace it.

Which leads to more questions. For example, what is the overall best way to provide fresh air for the stove? When I was installing the stove, I was thinking a lot about draft-avoidance. I was excited by an available accessory for the stove called a “fresh air intake kit”. As described, it would connect the stove’s air intake via a 4″ duct to a nearby hole you’d make in the wall. Thus, the stove would become a closed system, sucking air straight from outside, then exhausting it up the chimney. As I was very interested in avoiding drafts, I thought this sounded ideal for dodging drafts, so I ordered the kit with the stove. In reality, however, it turned out that the stove’s air intake is rather diffuse, and the “kit” did not provide a means of sealing a duct from outside right onto the stove– rather it just sort of directed the air in the general direction of the stove intake– more or less like cutting a hole in the wall near the stove to encourage the stove to breath from there, rather than somewhere else. This seemed silly to me and I never installed it.

Instead, then, our stove sucks air from– where? I’m not exactly sure. Probably a million little cracks in the windows, walls, doors, all at once. This, probably, is the best situation: not enough air coming from any one place to produce a strong draft, but enough intake to keep the fire happy. Yesterday, J. pointed out to me that the weather stripping below our kitchen door is worn, and no longer quite touches the threshold. She suggested we fix it to seal the gap. My initial instinct was, yes, that needs to be done. My second was, why bother?– if I seal that up, that just means the stove will suck more air in somewhere else. Indeed I could spend all my waking hours sealing the house up ever tighter, and but if I finally eliminate every last crack, what will the result be? A fire that won’t burn without a window open, and a house full of stale, toxic air.

So, my guess it that even with marginal wood stove use, we’re probably getting enough air replacement to keep the quality good. But I’m not really sure how much air we replace (working on an estimate for that). If it’s not enough, the ideal and elegant solution is a heat recovery ventilator— but with prices of these starting at about two years worth of heating costs for us, I won’t be running out to get one soon. A cheaper solution might just be a small air purifier with a charcoal filter.

Anyone have any suggestions on easy ways to determine whether our indoor air is good, bad, or unable to support life?

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“Surely your stove should be a closed system, drawing in outside air and venting its exhaust outside.”

Well, you’d think that makes a certain amount of sense, but it really isn’t do-able with this stove, or with the vast majority of others out there. For that matter, most conventional oil and gas furnaces don’t draw air directly from outside, either. I don’t think it makes much thermodynamic difference. It’s really just a question of the drafts (and, perhaps, the possibility that multiple air-sucking appliances could compete with each other– if we run the stove and the clothes dryer at once, the dryer might win… have to check into that!)

Just because conventional furnaces don’t do this doesn’t mean it isn’t a good idea. I believe that high-efficiency furnaces now do do this, and in fact exchange the heat between exhaust and intake to further maximize efficiency.

Not sure what thermodynamics has to do with it either (or why you raise that); it’s the draught and the sucking in of all that cold outside air that’s the problem. Driers are silly things too and it would be good to grab as much of that waste heat as possible.

“Not sure what thermodynamics has to do with it either (or why you raise that)”
I just meant that I don’t think it makes the house actually colder to draw air from inside rather than outside. Using cold outdoor air for combustion means the stove produces less heat, compared to using warm air; I think it’s a wash in terms of heat in the room per unit of fuel. But, the issue of moving air feeling colder than still air, that aspect I agree is a reasonably compelling argument.

“I believe that high-efficiency furnaces now do do this, and in fact exchange the heat between exhaust and intake”
Yes, that is pretty cool. My friends bough a (pricey) gas furnace that actually condenses the water vapor produced by combustion, reclaiming its heat of vaporization. (https://coldhousejournal.com/2010/01/28/a-tale-of-two-heating-systems/)

“Driers are silly things too and it would be good to grab as much of that waste heat as possible.”
I’ve mulled and experimented on this enormously, and have concluded that it’s all but impossible for a simple hack. The problem is that the majority of the “waste heat” of a drier is not in the form of heat, but in the form of liquid water turned to water vapor. In order to get that heat back, you have to condense the steam back to liquid, which requires application of cold. Since the process happens in a fast and furious way, you need a lot of cold, rapidly applied. Simple solutions, such as running the exhaust through a standard dehumidifier, are woefully underpowered (I’ve tried it). I conclude that it takes a purpose-built system, such as an actual condensing dryer, to do this job. And I haven’t even mentioned the issue of fine-particle lint dust, which you probably don’t want to be breathing regularly if you want to avoid byssinosis.

I think it makes the house colder because when you draw air from inside, you are drawing air from outside to replace that air, so the house gets colder. That and, as you point out, the moving-air thing.

Getting heat back out of the drier is tricky indeed, but the air itself is hot, even if a lot of the heat is sitting in that vaporized water. A metal exhaust tube will help transfer some of that heat without condensing the water, but who wants their drier in a space they live in anyway?

“Getting heat back out of the drier is tricky indeed, but the air itself is hot, even if a lot of the heat is sitting in that vaporized water. A metal exhaust tube will help transfer some of that heat without condensing the water.”

Hm– I half agree with you. Yes, the air itself is hot. But I don’t think you can get any of that heat back without condensing some water, purposefully or accidentally. The air coming out of the drier is hot and at close to 100% humidity. If you cool it at all before it gets outside, it will start to precipitate water. A metal exhaust tube through the house might reclaim a little heat, but then water would condense in it. Perhaps you could angle it to drip the water outside… but I envision the formation of damp lint gunk inside the tube. Anyway, estimations I did on this sort of thing last year lead to the conclusion that to reclaim any significant fraction of the drier heat in this way you’d need a tube hundreds of feet long, or with a very high mass of metal, or at extremely low temperatures, or some combination of all these impracticals…

The Europeans were clearly onto something when for centuries they built homes around masonry stoves and put the door to fuel the stove in an entryway that has lots of fresh air, but is closed off to the rooms of the house. Benjamin Franklin mused that the result is too little air circulation in the home since the entry way/mudroom works like a dedicated outside air intake.

My argument for closing off that leak under your door is so that when the stove is not burning, you are not getting that leak. Leaks happen not just because stoves suck air out, but because too much natural ventilation occurs between cracks down low and cracks up high.

“Benjamin Franklin mused that the result is too little air circulation in the home”
Thoreau wrote a similar rant against wood stoves in general (as opposed to fireplaces) in Walden. I’ll have to find it for some quotes…

Well if you’re not gonna bother punch a hole through the wall for the draft kit, you might at least plumb it to draw from the foulest indoor air – like the bathroom. Put the cat litter box in the bathroom and draw from directly from it.

Ha– great idea Leif. This reminds me that when we were first visited this house during the purchase process, the previous owner had his cat box in the cellar, but directly under an open grate (in fact, the only open grate) to the first floor. As if he was purposefully trying to smell it in the kitchen. Struck me as odd…

On further thought, wouldn’t it be great to hook up the wood stove intake to the cook stove vent (which in our case doesn’t go outside, or even filter anything– it just blows the air around.) Unfortunately it’s just a bit too far away to be practical…

While we know that it would be fine to feed the woodstove from the cookstove vent, it surely wouldn’t pass firecode. There’s certainly a presumption that cookstove vents are covered with a film of grease, and their exhaust is full of unburned grease & oil smokes.

The problem with a “closed system” is that not only do you have to seal the house up completely, but then you have to add some major dehumidification and artificial ventilation or you end up with major mold problems– which are now a terrible problem with all those nice newly built sealed homes. Frankly, give me an old house with overalll diffuse air leakages, a slightly higher heating bill and less stagnant air. No worries about CO backup when running both woodstove and dryer, etc., either.

As for the dryer itself– mine vents indoors through a simple plastic doobie I fill with water periodically. The water reservoir filters out all the lint and dust and just leaves a blast of nice humid air to circulate through the incredibly dry house air of winter. (I just open the window in summer.) Fine with me, but I do live in VT, which has very low humidity during the winter especially. It also warms up the otherwise pretty chilly bathroom quite nicely.

But in a house older than 20 years or so, by all means seal up the obvious air leaks. The house will “breathe” quite nicely anyway just through the way it’s constructed and the materials used in the walls.

There’s a nice healthy (and much more natural and less expensive) balance in an older home with good insulation. You trade a little bit of heating efficiency for a basically healthy air exchange in the liiving space, whatever heating method you use. The alternative is elaborate (and power-dependent and failure-prone) mechanical systems to dehumidify and exchange heat and ventilation and all that good stuff to go along with your perfectly sealed home.

Insulate your walls and attic, caulk up the noticeably leaky places that need to be caulked, and relax.

Jane– I generally agree with you. I think our 50-year-old house is Just Right in terms of tightness. The old Cold House (built c. 1870) was Not Tight Enough (you could hear the windows rattle on a windy night). And, ironically, the garage attached to our current house– built recently, with full Tyvek wrap and modern windows, is Too Tight. When water gets in, it never gets out– unless I leave a window open in the attic above it.

I’m going to have to disagree with your assertion that Vermont has “very low humidity during the winter especially”. In fact relative humidity in VT is pretty average, and doesn’t change dramatically with the seasons– in winter it’s a tad lower in morning, but December & January actually have considerably higher afternoon humidity than summertime. The “low humidity in winter” is solely a result of heating the air indoors. The cooler you keep the house, the higher the humidity will stay. See previous posts under the “Water Vapor” category…

Oh, you’re right, of course, about the humidity. I overstated. VT is hardly Arizona. But I’m originally from Boston and communicate regularly with wood-burning friends in the Pacific Northwest and the Southeast, so it’s bone dry here by comparison. One of the things that attracted me to living here in the first place was the bliss of dry rather than wet cold outdoors in winter. Also although VT is small, the climate varies quite a bit from place to place, so where I am is a good bit drier than Montpelier, not to mention there’s a difference between precipitation and humidity. But never mind.

I don’t have quite your tolerance for low temperatures, so I do run my small stove 24/7 during the winter, but I’m able to keep my main living space plenty warm enough for comfort during my waking hours on about 2 cords of wood a year.

One thing I have found is that all those small fixes they keep telling us to make– caulking up this or that, using insulated curtains, closing off unused rooms, etc.– add up very noticeably when you’re heating with wood in a way they don’t with central heating and thermostats. Every one of those small things adds a degree or two of warmth to the space from the same stove running at the same temperature in the same weather. And you don’t need to sit down with a calculator and your fuel bills and a lot of guesswork at the end of the year to figure out if it was worth doing.

You’re probably right about your cellar, given the way you guys have chosen to live. Mine is about the same temperature as yours during the winter, but since I like it a bit warmer in the house, I’ve filled in the wider cracks between the old floorboards and have thick rag rugs on much of the floor, and that does the job for me.

“you don’t need to sit down with a calculator and your fuel bills and a lot of guesswork at the end of the year”
Isn’t that great? You see the logs, you see how fast they’re moving, you know what’s going on.

“wood-burning friends in the Pacific Northwest and the Southeast”
They don’t even have winter in the Southeast– what are they burning wood for?? : )

I have the fervor of a religious convert to wood heat and I could go on about its manifold virtues nearly endlessly, as my patient but weary oil-addicted friends in the suburbs can attest. (don’t try me!)

As for the Southeast– are you kidding me? Why, it frequently, sometimes, gets down to a bone-chilling 40 degrees overnight in midwinter!

John said, “I think it makes the house colder because when you draw air from inside, you are drawing air from outside to replace that air, so the house gets colder. That and, as you point out, the moving-air thing.”

(How the heck do you reply to a comment directly and keep the thread going on this site?)

Yes, the house gets colder (or loses heat), but drawing air into the stove itself directly from outside lessens the heat the stove can put out because it has to use up energy heating up that cold air. So as Mr. Cold House says, it’s pretty much a wash. Outside air kits, or OAKs as they’re called, only make sense if you have a tightly sealed house.

You can see the same effect if you set up an electric fan to blow against the stove. It moves the warmer air farther around the room, but at the cost of lessening the operating temperature of the stove noticeably, and thus the warmth the stove puts out in the immediate vicinity. One of the down sides of using a wood stove. If you’ve got a big stove that reaches high temperatures, that’s not a problem, and is usually even a benefit. With a smaller stove and less heating capacity, like I have, you have to choose between real warmth in the immediate vicinicity of the stove and cold corners or less warmth but spread around farther.

Bottom line, there are only so many BTUs available from the load of wood you’re burning, and you choose how (and over how much time) they’re distributed but you can’t increase them beyond the content of the load you’ve got in the stove.

And you really don’t get a draft from cold moving air if you’ve caulked up the cracks. Insulated, well-caulked but unsealed houses have a much more diffuse, overall air exchange with the outside, not some whooshing draft coming in from particular spots. It seems counterintuitive that a solid and well insulated wall with no cracks or gaps or drafts could allow air exchange, but it does unless it’s sealed up tight with Tyvek or the like. Two of the four walls of my house are a foot thick, and also insulated. No drafts, believe it, but plenty of air exchange.

“but drawing air into the stove itself directly from outside lessens the heat the stove can put out because it has to use up energy heating up that cold air”

Yes, that’s what I was trying to say. Similarly if you set up your dryer to draw cold outside air, it wouldn’t suck cold air into the house– but it would use more energy, because it would have to heat the air more to get it to operating temperature.

“The main problem that can occur with stoves is the same as with fireplaces: other rooms can get really cold due to the in-suck of outside air, even though the room with the heat source feels fine.”

Well, gee, but to a massively smaller degree than with a fireplace, which functions like a big Hoover vacuum. The amount of air drawn into a modern stove is pretty small, even with the primary air control wide open. With it closed down most or all the way, which is the way you want to operate a woodstove 90 percent of the time, it’s a very small amount of air intake indeed.

I’m kinda dubious about the idea that a stove sucks enough cold air in through the walls to further chill the unheated space to any significant degree. Have you found any stats or figures or anything on that, or is it just a guess?

I will say that my unheated kitchen adjoining the stove room is no colder with the stove than it was when I was using central oil heat, nor are my unheated 2nd floor bedrooms. (The one over the stove is of course actually warmer.)

The main problem with using a stove for heat is that all the heat is being generated in one spot only, rather than distributed around baseboards or radiators. I think that perfectly well accounts for the reason it’s colder the farther away from the stove you are.

(Again, I’m talking about a decently but not spectacularly well insulated house with a fair amount of caulking in the usual places.)

I’m with Jane on this one. In spite of my fears before installing it, I haven’t noticed any serious air movement through the house from the stove. I might if we were wandering around barefoot… but we aren’t, in stove season.

“I’m kinda dubious about the idea that a stove sucks enough cold air in through the walls to further chill the unheated space to any significant degree.”

This gives me the idea to make some calculations. It will be a simple matter to determine how much air is used to combust a unit of wood, and how much heat is released– then consider some comparisons. Stay tuned! (In general I’m curious about the volume of gases up the chimney– it seems somewhat less than that produced by the drier, but really, no idea.)

Calculations– please do! You might also try the old trick of taking a lighted and smoking stick of incense around your stove when it’s burning and see if you can even find where the air is going in both with the primary air wide open and closed down entirely. (In case you don’t know this already, on most modern stoves it ain’t just in the front where the “doghouse” is.)

As for chimneys, I’ve seen some pretty awesome discussions/diagrams about the way air moves through a heated house in wintertime. Don’t think it was on Hearth.com. I’ll rummage around and see if I can find it.